1. Technical Field
This disclosure relates to a heat-resistant endless resin belt, a manufacturing method thereof and an image forming apparatus using the heat-resistant endless resin belt. The heat-resistant endless resin belt, serving as a fixing belt or an intermediate transfer belt, may be employed in complex machines such as a copying machine, a facsimile, a printer, etc.
2. Description of the Related Art
In recent years, an image forming apparatus for color printing has been employed extensively and high-speed printing has been developed.
Compared with black-and-white printing, when color printing is performed, because adhered toner is larger in amount at one time, high pressure and high temperature are applied to a recording paper when the recording paper is passed through nip portions between rollers.
When the high-speed printing is developed, further high heat resistance and mechanical strength are required of an endless belt, which passes through nip portions.
The endless belt, for example can be made from polyimide resin.
As one method for manufacturing the endless belt, there is proposed a method of performing cast molding to provide polyimide varnish on an outer peripheral surface of a cylinder made of a metal, and thereafter obtaining a polyimide endless belt by heating and imidizing the resultant polyimide varnish whereto the cast molding is performed. (For reference, see Japanese Application Publication Number Hei7-295396)
However, the above-mentioned manufacturing method may cause the following two main problems. One of the problems is that the imidation process is time-consuming, which incurs a cost increase. The other is that a plurality of molds are required because a new mold is needed whenever the dimensional standard is altered, which increases an initial cost.
In view of the above problems, a manufacturing technique of an endless resin belt at a low cost is proposed (For reference, see Japanese Application Publication Number Hei11-291348). To be specific, the polyimide endless belt is manufactured by adhering a non-thermoplastic polyimide film and a thermoplastic polyimide resin sheet together, whereafter the thermoplastic polyimide resin sheet is melted and bonded.
However, for one thing, it is difficult to make a film thickness of an overlapped portion of a resin film and sheet to be virtually the same as a non-overlapped portion of the film and sheet. For another, bonding without altering a hardness and a surface property of the two sheets cannot be easily executed in practice. Consequently, when a fixed image is formed by the finally manufactured resin belt, there is a problem that a streaky defective image is generated.
It is also proposed that an endless resin belt can be manufactured via forming a groove-like protrusion and recess in a thickness direction at both ends of a polyimide resin sheet, whereafter fitting the protrusion in the recess and bonding (For reference, see Japanese Application Publication Number Hei10-698).
The problem with the aforementioned resin belt which is processed at both ends and is joined by an adhesive (a joined belt will be used hereinafter) lies in the circumferential length accuracy of the belt, which remains to be solved hereafter.
Although the circumferential length accuracy may differ according to specification standards of the image forming apparatus whereto the resin belt is applied and a diameter of the resin belt, the circumferential length accuracy of the resin belt is preferably approximately equal to or less than 0.05 mm.
In particular, when the circumferential length accuracy differs in a width direction of the belt (the axial direction wherein the endless resin belt is stretched), the belt skews, which is transcribed by the recording paper upon printing. Consequently, wrinkling is present on the recording paper.
In addition, when the image forming apparatus is in operation and the belt is in contact with a heat roller and a pressure roller, a perimeter distribution poses a problem of belt skew, which probably causes local shaving of the belt inner surface.
Thus, in order to maintain image quality, it is important to enhance the circumferential length accuracy, namely flatness, in the axial direction of the heat-resistant resin belt which constitutes the image forming apparatus.
There is proposed a flatness measuring method to measure the flatness of the heat-resistant endless belt (For reference, see Japanese Application Publication Number 2007-58059). To be specific, the seamless belt is stretched by a predetermined tensile force via at least two rollers and is set in a non-rotating state. The flatness is measured via surface scanning. In accordance with this method, in order to solve the problem arising at the time of image forming, the flatness is limited to be equal to or less than 5 mm throughout the seamless belt.
However, in the case of the endless resin belt joined at the both ends, various flatness patterns arise, which is rather different from the case with the seamless belt. Therefore, even if the flatness is limited to be equal to or less than 5 mm, the problem that the wrinkling is unalterably transcribed to the recording paper remains unsolved.